Magnetostrictive Dental Scalers Paul Bartlett Wolfson Centre for

Magnetostrictive Dental Scalers Paul Bartlett Wolfson Centre for Magnetics 29 March 2007

Outline of Presentation • • Description of magnetostriction Introduction to dental scalers Description of the main dental scaler components Wolfson Centre work on magnetostrictive materials characterisation • Wolfson Centre work on the dynamic properties of dental scalers • Wolfson Centre Finite Element Modelling • Wolfson Centre work on new dental scalers

What is Magnetostriction? • When a magnetic field is applied to a ferromagnetic material (eg iron), the series of domains rotate to partially align themselves to the magnetic field direction. • This results change in length (Δl) known as the strain (e). Source: MTS Sensors

What is an Ultrasonic Dental Scaler • Usually a magnetostrictive device that is used to remove ‘calculus’ from teeth • Utilises a combination of ultrasonic vibrational abrasion and cavitation • Comprises of the: – Power Supply – Handle – Insert • Has interchangeable inserts’ with different geometry tips Dentspy International

Analysis of typical dental scalers Drive Coil Tip Magnetostrictive Stack Grip Handle Sensing coil (under secondary drive coil) • • X-Ray EDS analysis showed the laminates were of high purity nickel Drive signal ramps up to 40 k. Hz when trying to identify resonance Drive system seeks for a resonance to lock on to This is done by identifying a phase change in a pick-up coil signal

DC Magnetostiction Characterisation Magnetostrictive sample Pre-stress rig DC drive coil Sample holder Opened Sample holder DC characterisation test rig that was used to obtain strain Vs. applied field data for various temperatures and pre-stresses for strips of magnetostrictive material.

Example DC Strain-H plots

AC Material Measurements • A characterisation rig has been developed for the AC analysis of sample strips of candidate materials. Single-point laser vibrometer head AC drive coil with strip of material inserted

Example dynamic analysis of strips via laser vibrometry Ist, 2 nd and 3 rd Harmonics for a single drive field value. 2 nd harmonic dominant 2 nd Harmonic at differing drive fields

Analysis of Current Dental Scalers Dynamic analysis of current inserts Resonance sensing system Phase sensing coil Phase sign change in region of 30 k. Hz resonance

Production of scalers and coils Standard Insert Prototype Insert Modified Drive Coil Stacks were produced of the candidate materials and were insulated against eddy current effects with a spray enamel paint. This paint has an operating temperature of up to 425 o C. A Dentsply drive coil had to be modified to take the larger diameter stacks without damaging the coil systems.

Finite Element Modelling • Simple 2 d and 3 d models produced • Dynamic model shown • Multi-element stack model also produced

Dynamic FE Analysis 3 d Finite Element mechanical model Mechanical modelling showed; • That there were flexural modes in the region of previously measured resonances • That modelled tip vibrations closely matched actual results obtained by the Birmingham University team • This suggests that modelling could be used for other tip-shape designs Scanning laser vibrometer data of a tip (University of Birmingham) Note the similarity of shape of the modelled and real tip vibrations

Scaler Failure Issue Modelled flexural mode in the region of the insert resonance. (Vibration mode magnitude exaggerated) So-called ‘broken’ insert. Compare the form of the stack distortion to that of the model above

What was covered in the presentation • • Description of magnetostriction Introduction to dental scalers Description of the main dental scaler components Wolfson Centre work on magnetostrictive materials characterisation • Wolfson Centre work on the dynamic properties of dental scalers • Wolfson Centre Finite Element Modelling • Wolfson Centre work on new dental scalers